Fresnel lenses are examples of optical parts having deflecting or reflecting structured optical surfaces. These structured optical surfaces often include a multitude of juxtaposed sections, each section having, for instance, one or more circumscribed facets or the like. Fresnel lenses can be made much thinner than conventional non-structured lenses of equivalent power. They are used in a wide range of applications, such as for visual and image displays, solar light concentrators, automotive and other lighting devices and image capturing systems, to just name a few. Other kinds of optical parts having similar optical deflecting or reflecting structured surfaces exist as well.
Examples of various optical parts can be found for instance in U.S. Pat. No. 4,170,616, U.S. Pat. No. 6,171,095, U.S. Pat. No. 6,746,634, U.S. Pat. No. 7,102,820, U.S. Pat. No. 7,762,165, U.S. Pat. No. 8,184,386, JP-2011083952 and US-2010/0323105.
Injection molding is one of the manufacturing processes that can be used to manufacture optical parts having structured optical surfaces. It uses a molten material, generally a molten plastic material, to form the optical parts and their structured optical surfaces. During the injection molding process, the molten material is injected at relatively high temperatures and using high injection pressures. Gas present in the mold cavity is vented out as the molten material is injected therein. Examples can be found for instance in U.S. Pat. No. 6,367,765, U.S. Pat. No. 6,527,538, U.S. Pat. No. 7,862,752 and US-2008/0018008.
High pressures are maintained when the molten material cools inside the mold cavity to form the structured optical surface as accurately as possible. The molten material is then pressed again a negative of the structured surface until it solidifies.
Injection molding has some advantages over other manufacturing processes but injection molding issues can have a negative impact on the quality of the resulting optical parts, for instance in terms surface accuracy. The structured optical surfaces of some optical parts can be relatively complex to manufacture, particularly when they have a multitude of sections with facets having very sharp angles between them.
One of the difficulties in injection molding optical parts having structured optical surfaces is due to the presence of small residual gas pockets trapped at some locations in the mold cavity during the molding. Small amounts of the gases always tend to remain within the mold cavity, for instance where two adjacent sections of the structured surface meet with an acute angle and where the radius is very small. Gas pockets can also form where two adjacent sections meet at a right or obtuse angle. There are often a large number of locations where gas pockets can form on each optical part and they prevent the molten material from reaching some very small portions of the mold cavity.
Several applications require stringent tolerances and a very high accuracy of the structured optical surface, for instance of the shape, curvatures and/or angles. This high accuracy can mitigate diffraction of white light or monochromatic light, for instance. However, the very small radius of some of these designs can be very difficult to obtain using these known injection molding processes because of the trapped gas pockets.
Another challenge is the need of performing faster maintenance the mold inserts. Providing injection molding apparatuses with very complex gas venting arrangements can lead to costly and time consuming maintenance operations, especially when most of the gas venting circuit is deeply embedded into the body of the parts. A gas venting circuit that cannot be cleaned easily and/or completely will be less efficient and this may eventually impact the quality of the molded optical parts.
Clearly, room for improvements still exists in this area of technology.